Typical knee prostheses include a tibial component, a femoral component, and a patellar component. The femoral component generally includes a pair of spaced apart condylar portions, the superior surfaces of which articulate with a portion of the tibial component. A femoral stem assembly, used to provide stability to the replaced knee joint, seats within the medullary canal of a distal portion of a femur, and is typically coupled to the femoral component by specialized coupling devices, such as a collar and bolt. Some prosthetic knee joints include a structure known as a Morse taper post that extends from the inferior surface of the femoral component to mate with a femoral sleeve that is securable to the femoral stem assembly. The femoral component may include a boss having a slot for receiving the components of the modular junction.
Knee replacement surgery requires the replacement of the distal end of the femur and the proximal end of the tibia. Implant loosening, infection, and device wear are well-documented failure modes of primary knee arthroplasty. In cases where the primary implants fail, a secondary operation is required to replace the faulty device. The factors associated with device failure, including infection and osteolysis, often lead to a deterioration of bone quality in proximity to the implanted knee replacement devices. Upon removal of the faulty device, large bone defects are often encountered on both the tibia and the femur. These defects are often characterized by large cavitary voids along with cortical rim defects. Traditionally, treatment of these defects required the removal of a large amount of stable cortical bone to facilitate the implantation of a metal replacement for the bone defect.
Revision knee arthroplasty has existed since the first primary knees failed and the problem encountered due to bone loss upon revision is therefore not a new problem. Noiles et. al., U.S. Pat. No. 4,846,839 (“the 839 patent”), disclose a method for affixing a prosthesis to bone that incorporates flanges being placed on the femoral stem conforming to the anatomy of the femoral canal. The geometry of the device transfers stresses to the bone in a manner that generally corresponds to the types of stress transfers that occur in natural bone. The prior art also mentions coating or roughened surfaces that could be applied to stepped tibial sleeves to improve fixation with the bone.
Blaylock et al., U.S. Published Patent Application No. 200410162619 (“the 619 application”), describe a femoral augment system to replace bone defects as described in US 200410172137. The proposed invention incorporates U-shaped augments made of a highly porous tantalum material. The intent of the device is said be minimization of bone loss while providing a stable basis for a femoral implant. The device, however, does not incorporate the stepped design and therefore does not optimally transfer stress from the implant to the bone. The device also does not contain a means for mechanically fixing the femur to the femoral augment. The femur must be cemented to the femoral augment that provides for a decreased rotational stability compared to a mechanical lock.
While much research has been invested into devices that can be used when a primary knee implant fails, there are still important improvements that are needed. Areas of needed improvements include improved transfer of stresses to the bone in a manner that generally corresponds to the types of stress transfers that occur in natural bone and improved rotational stability.